(a) Field of the Invention
The present invention relates to a promoter useful for purifying the exhaust gases discharged from the internal combustion engines of, for instance, motorcars as well as a method for the preparation thereof. More specifically, the present invention pertains to a promoter for purifying the internal combustion engine exhaust gas as well as a method for preparing the same, wherein a cerium-containing double oxide is substituted for cerium oxide which has conventionally been used as a promoter for purifying the internal combustion engine exhaust gas and the former is supported on particulate aluminum oxide to thus improve the heat resistance of the resulting promoter and to hence prevent any reduction of the quality thereof due to thermal deterioration.
(b) Description of the Prior Art
As a catalyst for purifying the exhaust gas discharged from internal combustion engines of, for instance, motorcars, there have been used, for instance, those obtained by finely distributing noble metals such as platinum, rhodium and palladium on a carrier having a large specific surface area such as alumina. These noble metals can serve to oxidize and convert the hydrocarbons present in the exhaust gas into carbon dioxide and water and to oxidize and convert the carbon monoxide into carbon dioxide, while they also have an ability of reducing the nitrogen oxides present in the exhaust gas into nitrogen gas. In other words, they can exert a catalytic effect on both oxidation and reducing reactions to thus permit the simultaneous removal of these three components of hydrocarbons, carbon monoxide and nitrogen oxides present in the exhaust gas. It has been well known that the ratio of air to fuel (or air-fuel ratio) should be maintained constant (at the theoretical air-fuel ratio) in order to ensure this effective catalytic effect on the both reactions simultaneously.
However, the internal combustion engines of, for instance, motorcars are apt to cause rapid change in the number of revolutions and the air-fuel ratio greatly varies depending on the traveling conditions such as the accelerated conditions, reduced speed conditions and low speed-traveling and high speed-traveling conditions. For this reason, oxygen concentration changes in the exhaust gas are continuously detected or monitored using an oxygen sensor to thus always control the amount of fuel to be supplied to an engine so that the air-fuel ratio is always kept constant.
On the other hand, it is common to use a combination of a noble metal catalyst and a promoter for the purpose of preventing any reduction of the purifying ability of the catalyst due to the change in the air-fuel ratio by the chemical action of the catalyst by itself. As such a promoter, there has been used, for instance, cerium oxide. Cerium oxide has such characteristic properties that it can release and/or absorb the oxygen attached thereto and the lattice oxygen present in the cerium oxide crystals depending on the level of the oxygen partial pressure in the exhaust gas. Accordingly, if the exhaust gas has a reducing ability, cerium oxide undergoes release of oxygen (CeO2xe2x86x92CeO2xe2x88x92x+0.5xc3x97O2) and supplies oxygen to the exhaust gas to cause an oxidation reaction. On the other hand, if the exhaust gas has an oxidation ability, cerium oxide conversely incorporates oxygen into the oxygen-deficient sites of the crystals (CeO2xe2x88x92x+0.5xc3x97O2xe2x86x92CeO2) to reduce the oxygen concentration in the exhaust gas and to thus cause a reducing reaction. As has been described above, cerium oxide serves as a buffering agent which can adjust any change in the extent of oxidizing and/or reducing ability of the exhaust gas and can thus maintain purifying quality of the catalyst.
However, the promoter for purifying the exhaust gas discharged from the internal combustion engines is exposed to high temperature gases generated from the engines over a long period of time and therefore, it is apt to be easily deteriorated in its ability. In particular, cerium oxide has a low heat resistance and accordingly, undergoes sintering upon exposure to high temperature gases and the specific surface area thereof is correspondingly reduced. As a result, the initial characteristic properties peculiar to the promoter is greatly impaired or reduced. For this reason, there have been proposed various means for eliminating the foregoing drawbacks of cerium oxide as a promoter, for instance, a method in which other elements such as zirconium are added to cerium oxide to improve the heat stability thereof and a method wherein the lattice-constant of cerium oxide crystals is enlarged by the incorporation of other elements into the cerium oxide crystals to thus improve the stability of the cubic system thereof. However, there has not yet been obtained any satisfactory results.
In addition, there have widely been used a technique, in which a promoter is prepared by mixing cerium oxide with aluminum oxide (such as xcex1-, xcex3- and xcex8-type ones). In this case, the coexistence of aluminum oxide surely has an effect of physically suppressing any sintering of the resulting mixture, but cannot suppress, at all, any sintering of cerium oxide per se.
Accordingly, it is an object of the present invention to provide a promoter for purifying the exhaust gas discharged from internal combustion engines, whose heat resistance is improved and whose quality reduction due to thermal deterioration can be suppressed, by the use of a cerium-containing double oxide in place of cerium oxide conventionally used.
As has been discussed above in detail, any sintering of cerium oxide per se cannot be suppressed even if a promoter is prepared by mixing cerium oxide with aluminum oxide. However, as a result of various repeated investigations of the inventors of this invention, it has been found that a cerium-containing double oxide having excellent heat resistance and free of any sintering can be prepared by bringing an aqueous solution containing a water-soluble salt of cerium and a water-soluble salt of a specific element, dissolved therein and keeping contact with particulate aluminum oxide, into contact with a specific precipitant to cause a reaction therebetween and to thus deposit the reaction product on the particulate aluminum oxide; and then firing the aluminum oxide to which the reaction product is adhered, to thus give the double oxide constituted by cerium, the specific element and oxygen supported on the particulate aluminum oxide.
Under such circumstances, the inventors of this invention have conducted various studies to achieve the foregoing object, have found that a promoter can be prepared by using a cerium-containing double oxide instead of cerium oxide and supporting the double oxide on the particulate high-heat-resistant aluminum oxide, the resulting promoter does not cause any reduction of its initial activity as a promoter even if it is exposed to a high temperature over a long period of time and can maintain its high quality and thus have completed the present invention on the basis of the foregoing findings.
According to an aspect of the present invention, there is thus provided a promoter for purifying internal combustion engine exhaust gas, which comprises (A) particulate aluminum oxide and (B) a double oxide of (i) cerium and (ii) at least one member selected from the group consisting of zirconium, yttrium, strontium, barium and rare earth elements, supported on the aluminum oxide.
According to another aspect of the present invention, there is provided a method for preparing a promoter for purifying internal combustion engine exhaust gas, which comprises the steps of mixing a dispersion, which contains (i) a water-soluble salt of cerium and (ii) a water-soluble salt of at least one member selected from the group consisting of zirconium, yttrium, strontium, barium and rare earth elements, dissolved therein and particulate aluminum oxide dispersed therein, with an aqueous solution of ammonium hydrogen carbonate; then reacting them to deposit the reaction product on the particulate aluminum oxide; and firing the particulate aluminum oxide to which the reaction product is adhered to thus give a promoter which comprises the particulate aluminum oxide and a double oxide of (i) cerium and (ii) at least one member selected from the group consisting of zirconium, yttrium, strontium, barium and rare earth elements, supported on the aluminum oxide.
According to a third aspect of the present invention, there is provided a method for preparing a promoter for purifying internal combustion engine exhaust gas, which comprises the steps of adhering, onto particulate aluminum oxide, an aqueous solution, which contains (i) a water-soluble salt of cerium and (ii) a water-soluble salt of at least one member selected from the group consisting of zirconium, yttrium, strontium, barium and rare earth elements, dissolved therein; bringing the particulate aluminum oxide provided thereon with the aqueous solution into contact with an aqueous solution of ammonium hydrogen carbonate to cause a reaction between these solutions and to thus deposit the reaction product on the particulate aluminum oxide; and then firing the particulate aluminum oxide to which the reaction product is adhered to thus give a promoter which comprises the particulate aluminum oxide and a double oxide of (i) cerium and (ii) at least one member selected from the group consisting of zirconium, yttrium, strontium, barium and rare earth elements, supported on the aluminum oxide.
The particulate aluminum oxide used in the present invention is preferably one having a high specific surface area and high thermal stability and specific examples of such aluminum oxides include xcex1-alumina, xcex8-alumina, xcex3-alumina and alumina to which different kinds of elements (such as alkaline earth metals and silicon) are added. The particle size of these particulate aluminum oxides is not limited to any specific range, but is preferably not less than 10 xcexcm in order to prevent any cohesion of aluminum oxide powder and to ensure high dispersibility, even when the powder supports the double oxide on the surface thereof.
Regarding double oxides, an effect of improving the heat resistance of the cerium oxide has been observed for a variety of double oxides constituted by cerium, various kinds of other elements and oxygen, but higher heat resistance is, in particular, observed for the double oxides of (i) cerium and (ii) at least one member selected from the group consisting of zirconium, yttrium, strontium, barium and rare earth elements (such as lanthanum, praseodymium, neodymium and ytterbium) (in this specification, these elements other than cerium will sometimes be referred to as xe2x80x9cadditive element(s)xe2x80x9d). In the present invention, cerium oxide, which has conventionally been used as a promoter for purifying the internal combustion engine exhaust gas, is used in the form of such a double oxide. Therefore, this permits the substantial improvement of the heat resistance of the promoter and the considerable inhibition of any quality reduction thereof due to thermal deterioration.
The component ratio of the particulate aluminum oxide to the double oxide and that of the cerium to the additive element present in the double oxide are not limited to any specific range, respectively in the present invention. However, if the relative amount of cerium in the promoter is reduced, the resulting promoter has a tendency of exhibiting an insufficient effect. Contrary to this, if the relative amount of cerium in the promoter increases, there is observed such a tendency that the cerium is present not only in the form of a double oxide, but also in the form of cerium oxide, or that the amount thereof is too large to effectively deposit these compounds on the particulate aluminum oxide and this never results in the effect in proportion to the increase in the amount of cerium. Therefore, the component ratio of the particulate aluminum oxide to the double oxide is preferably on the order of about 1:0.5 to 1:4, as expressed in terms of weight ratio. On the other hand, the component ratio (weight ratio) of the cerium to the additive element present in the double oxide preferably falls within the range of from about 1:0.1 to 0.1:1 and more preferably 1:0.2 to 0.2:1, as expressed in terms of the reduced amount of oxides.
In the production method according to the present invention, a dispersion, which contains (i) a water-soluble salt of cerium and (ii) a water-soluble salt of at least one additive element, dissolved therein and particulate aluminum oxide dispersed therein is mixed with an aqueous solution of ammonium hydrogen carbonate and then reacted together. Such a dispersion may, for instance, be one prepared by dissolving a water-soluble salt of cerium and a water-soluble salt of at least one additive element in water and then dispersing particulate aluminum oxide in the resulting aqueous solution or one prepared by adding water to a mixture of a water-soluble salt of cerium, a water-soluble salt of at least one additive element and particulate aluminum oxide. After such an aqueous solution sufficiently penetrates even into fine pores present in the particulate aluminum oxide, the dispersion is mixed with the aqueous solution of ammonium hydrogen carbonate as a precipitant to thus cause a reaction between them. In this respect, these dispersion and aqueous solution may be mixed together by adding the former to the latter or by adding the latter to the former.
Alternatively, it is also possible, in the production method according to the present invention, to adhere, to the particulate aluminum oxide, an aqueous solution containing (i) a water-soluble salt of cerium and (ii) a water-soluble salt of at least one member selected from the group consisting of zirconium, yttrium, strontium, barium and rare earth metals; and to then bring the particulate aluminum oxide, to which the aqueous solution is adhered, into contact with an aqueous solution of ammonium hydrogen carbonate to thus cause a reaction of these solutions.
The particulate aluminum oxide carrying the aqueous solution may be prepared by dispersing the particulate aluminum oxide in the aqueous solution and after the aqueous solution completely or sufficiently penetrates into fine pores of the aluminum oxide particles, separating the particulate aluminum oxide through, for instance, filtration; or alternatively by placing the particulate aluminum oxide on a filtering medium and then adding the aqueous solution from the top of the filtering medium to pass the same through the particulate aluminum oxide layer and the filtering medium.
The aluminum oxide particles, to which the aqueous solution is adhered, may be brought into contact with the aqueous solution of ammonium hydrogen carbonate by, for instance, dispersing the former in the aqueous ammonium hydrogen carbonate solution or by placing the aluminum oxide particles, to which the aqueous solution is adhered, on a filtering medium and then running the aqueous ammonium hydrogen carbonate solution through the layer of the aluminum oxide particles.
In case where the aqueous solution is adhered to the particulate aluminum oxide, the aluminum oxide particles containing the aqueous solution is then brought into contact with the aqueous ammonium hydrogen carbonate solution to thus cause a reaction of these aqueous solutions, a problem arises, such that most of the reaction product of cerium and the additive element with ammonium hydrogen carbonate is adhered to the particulate aluminum oxide, but only a small amount of the reaction product is adhered to the aluminum oxide particles, since only a small quantity of the aqueous solution can be adhered to the particles. However, this problem can be eliminated to some extent, by increasing the concentrations of the water-soluble salts of cerium and the additive element to be added to the aqueous solution.
In either of the foregoing reactions, the reaction product containing cerium and the additive elements is deposited on the particulate aluminum oxide. The reaction product thus obtained may be a double salt, a double oxide or a mixture thereof depending on the reaction conditions selected. For instance, when the dispersion is added to the aqueous ammonium hydrogen carbonate solution, the reaction has a tendency to form a double oxide, while when the carbonate aqueous solution is added to the dispersion, the reaction is liable to form a double salt.
The kinds of the water-soluble salts used in the methods are not restricted to any specific one, but it is preferred to use nitrates, for the purpose of inhibiting any possible adverse effect of anionic impurities. Moreover, a variety of alkaline solutions are inspected for their effect as precipitants and compared with each other and as a result, it has been found that a good result can be ensured when using ammonium hydrogen carbonate.
In the production method according to the present invention, the relative amounts of the particulate aluminum oxide, the water-soluble salt of cerium and the water-soluble salt of the additive element are not restricted to any particular range. However, it is preferred to adjust the amounts of these components in such a manner that the component ratio of the particulate aluminum oxide to the double oxide in the promoter finally prepared and the component ratio of cerium to the additive element in the double oxide should fall within the preferred ranges specified above, respectively. In addition, the reaction temperature is not likewise restricted to any particular range, but it has been found that the reaction should be carried out with heating rather than at room temperature in order to obtain a double oxide showing high heat resistance.
The particulate aluminum oxide, to which the reaction product is adhered and which is prepared by the foregoing method, is then separated by, for instance, filtration, washed and then subjected to firing. If the reaction product is a double salt, the water and the carbonate are decomposed through the firing operation of the double salt to thus give a double oxide and the resulting double oxide is deposited on the particulate aluminum oxide. On the other hand, if the reaction product is a double oxide, the double oxide thus formed is supported on the particulate aluminum oxide during the firing step. In this respect, the firing temperature is not limited to any specific range and it is in general sufficient to fire the reaction product at a temperature ranging from 500 to 600xc2x0 C. However, it is also possible to fire the product at a higher temperature and this permits the production of a promoter having improved thermal stability.
In the promoter for purifying the exhaust gas discharged from the internal combustion engines according to the present invention, the cerium-containing double salt is substituted for the cerium oxide conventionally used, unlike the conventionally used powdery mixture of cerium oxide and aluminum oxide and the double oxide is firmly adhered to the particulate aluminum oxide. For this reason, the resulting promoter has improved heat resistance, suppress any sintering even when it is exposed to a high temperature over a long period of time and can substantially prevent any quality reduction due to thermal deterioration. Furthermore, the particulate aluminum oxide is covered with the double oxide and therefore, the probability that the promoter comes in contact with exhaust gases.